A technology for accurately estimating an internal state of a secondary battery such as a state of charge (SOC) and a state of health (SOH) is important for electric vehicles, railroad vehicles, installed power storage systems, and other systems to use the secondary battery efficiently.
A known example of related art for estimating the SOC is a current integration method for estimating a current SOC by using an initial value of the SOC and an integrated value of a measured current. Further, another known example is an open circuit voltage (OCV) estimation method for estimating the current SOC by estimating an OCV of the battery based on a battery equivalent circuit model and using an OCV-SOC curve.
The current integration method and the OCV estimation method have different characteristics. Specifically, the current integration method can accurately follow short-term change of the SOC, but is susceptible to influences of errors of parameters that are an initial electricity quantity, the SOH, and a current offset. In particular, the error of the current offset is integrated, and as a result, the accuracy of estimating the SOC deteriorates with passage of time.
On the other hand, the OCV estimation method mainly uses a measured voltage to estimate the SOC, and thus the error of each parameter is not accumulated unlike the current integration method. However, it is known that the OCV estimation method is strongly susceptible to influences of errors of equivalent circuit parameters and the measured voltage, and thus the short-term estimation accuracy is poor due to, for example, occurrence of an outlier of the SOC estimation value.
Under such circumstances, there have been proposed numerous SOC estimation methods involving suitably using the current integration method and the OCV estimation method in combination to achieve advantages of both methods while compensating for disadvantages thereof.
A specific example of the SOC estimation method is a technology of calculating a final SOC estimation value by weighting and combining an SOC estimation value estimated with the current integration method and an SOC estimation value estimated with the OCV estimation method depending on usage of the battery (for example, refer to Patent Literature 1).
Further, as described above, the technology for accurately estimating the SOH is also important, and accurate estimation of the SOH enables replacement time of the secondary battery to be grasped appropriately, and achieves improvement of the accuracy of estimating the SOC.
A specific example of the SOH estimation method is a technology of calculating an SOH estimation value (that is, battery capacity estimation value) that is not susceptible to the current integration error by using SOC variation amounts of the current integration method and the OCV estimation method in a period in which a charge/discharge current exceeds a predetermined threshold value (for example, refer to Patent Literature 2).